1. Field of the Invention
The present invention relates to methods for generated selected surface structures on silicon crystal wafer by means of neutron irradiation, and in particular to a method for generating such regions with adjustable uniform doping suitable for use in manufacturing power thyristors.
2. Description of the Prior Art
Production of silicon crystals with uniform n-conductivity by irradiation with thermal neutrons is known in the art. The doping ensues according to the nuclear reaction Si.sup.30 (n, .gamma.) Si.sup.31 .beta..sup.- P.sup.31. The natural isotope Si.sup.30 present in the silicon wafer is converted into the unstable isotope Si.sup.31 by absorbing a thermal neutron and emitting gamma radiation. The unstable Si.sup.31 is converted into the stable phosphorous isotope P.sup.31 upon emission of .beta..sup.- radiation with a half-life of 2.62 hours. In undertaking such irradiation, it is also known to cover selected regions on the silicon crystal wafer with neutron-absorbing materials.
A method for manufacturing pn-junctions in semiconductor components by doping using nuclear transformation is described in U.S. Pat. No. 3,255,050. A semiconductor crystal wafer of a selected conductivity is covered with absorber material except for those regions to be redoped, thus exposing the uncovered regions to neutron irradiation. After the radiation damage has cured, sharp pn-junctions are obtained in the crystal wafer. Boron and cadmium are used as absorber materials.
German Pat. No. 25 62 621 describes a method for manufacturing n-doped silicon single crystals having a dish-shaped profile of the specific resistance in the radial direction by means of irradiation with thermal neutons wherein the silicon single crystal is rotated during irradiation and is exposed to a more intense or less attenuated beam of thermal neutrons toward its center as opposed to its edge. The different irradiation is achieved, for example, by the use of a slit-shaped diaphragm of neutron absorbing material, for example, a cadmium sheet or boron glass which has a variable slit width. The absorber sheet is disposed in front of the silicon monocrystal. The silicon monocrystal is provided with a non-uniform doping over a large area in this manner, and is suitable for manufacturing large area, high-inhibiting power thyristors.